Integrated circuits can be very complex and subject to a variety of different failures caused by flaws in the manufacturing process. Accordingly, they are thoroughly tested before being sold. Testing equipment is also used during development to ensure that a theoretical design performs well in practice and to ensure that the manufacturing processes are producing functional parts. During test and debug processes, probes are applied to various nodes that are on the surface or near the surface of the integrated circuit. The probes measure electrical characteristics of the integrated circuit at different locations. The measured device parameters may include voltage levels, timing information, current levels and thermal information.
While there are many possibilities for directly probing the metal interconnects on a chip. Laser probing allows even more data to be collected without affecting the operation of the chip. A laser beam is focused through the back side of the silicon of a DUT (Device Under Test). The beam is reflected from active regions of the DUT and directed to a detector. By measuring the changes in the amplitude of the reflected beam, the detector indicates changes in the charges within a PN junction region under the probe caused by the operation of the P-N junction region. Using repeated measurements, high frequency voltage waveforms from the P-N junction region can also be measured. In some cases a part of the probe laser is reflected from a metal region and used as a reference to compare with the light reflected from the P-N junction.
Laser voltage probing allows circuits to be probed without contact and it allows circuits that are buried below several silicon layers to be probed. The amplitude difference measured by the detector is caused by a change in the charge concentration in a P/N junction of, for example, a transistor during operation.
The amount of the laser beam absorbed by the P/N junction changes as the electric field within the P/N junction changes. The electric field is determined by the electric charge within the junction. As the absorption changes, the amount of light reflected back from the junction changes. The changes in the reflected light can be used to characterize the charge and therefore, the electric field, and therefore the voltage at the junction.